Pythium spp.

(Root rots and losses)

Pythium spp. occurs on seedlings in the nursery. When weather conditions are favourable for the pathogen  development some Pythium species can cause lesions on roots and collar of mature tobacco in the field (figures 1 and 2). We have never observed such attacks after planting in France. They seem to occur especially in the tropics during the rainy season or after excessive irrigation.

Note that only the observation of oospores (and sometimes other structures, figure 3) in infected tissues, or the development of isolates in an artificial media, allows the diagnosis of this disease with certainty (see Nursery and  planting diseases).

Further information on these pathogens is available in the section of parasitic fungi of root, collar and stem, Different oomycetes.